Defining the region of interest of the knee for perioperative volumetric assessment with a portable 3D scanner in orthopedic and trauma surgery

Background The aim of this study was to characterize three regions of interest (ROI) around the knee with a portable 3D scanner (Artec 3D scanner EVA). Soft tissue topography assessment with an optimized, precise, and reproducible method may assist surgeons when managing soft tissue swelling in the post traumatic setting. Methods 12 healthy volunteers (24 legs, 7 women, 5 man) were included in this study. The patient cohort showed a mean age of 27.1 years (SD±3), a mean weight of 70 kg (SD±13) and a mean height of 171 cm (SD±8.8). All scans were recorded by the same examiner in the same room and with the same scanner (Artec, 3 D scanner EVA). Three volume regions of interest (ROI) were defined: the distal femur (circumference measured between the of superior extent of the patella to 10 cm proximal), the knee joint (measured from the top of the patella to the tibial tuberosity) and the proximal tibia (tibial tuberosity to 10 cm distal). Results The mean volume of the right leg was 3.901 l (I. distal femur: 1.63 l, knee joint: 1.33 l, proximal tibia: 1.10 l) and mean volume of the left leg was 3.910 l (I. distal femur: 1.66 l, knee joint: 1.34 l, proximal tibia: 1.12 l). The volume difference between the right and left leg was 0.094 l (SD ± 0.083 l) The Wilcoxon-Mann-Whitney test showed no significant differences of the volumes between the right and left leg. Conclusions This study demonstrates that portable 3D scanning could be an accurate and reliable tool for orthopedics and trauma surgeons. Based on the ROIs of this pilot study, further studies are needed to test the significance for clinical applications for patients with an injured knee.


Introduction
Soft tissue swelling and edema are frequently encountered when managing orthopedic pathology [1]. The knee is prone to swelling secondary to tendinitis, arthritis, as well as any type of trauma such as fractures or ligament injuries. Severe swelling secondary to proximal tibial or distal femoral fractures precludes immediate operative intervention with open reduction and internal fixation and wrong timing of surgery being highly correlated with substantial soft tissue complications [2]. Therefore, a staged soft tissue management algorithm that begins with closed reduction and external fixation that is followed by open reduction and internal fixation is mandatory [2]. Tape and water displacement methods represent valid tools in the assessment of soft tissue swelling and yet are difficult to standardize and are subject to operator variability [3]. A gold standard for perioperative swelling characterization does not exist, which introduces a substantial subjective component regarding the timing of management for high energy injuries [2]. A portable three-dimensional (3D) scanner has been developed that measures the volume of a region of interest (ROI) [3,4]. Previous studies have demonstrated that 3D scanning can efficiently achieve objective and reproducible measurements that correlate well with previously established tape measurement and water displacement methods [4]. The capabilities as they relate to perioperative traumatic soft tissue management have not been evaluated. The aim of this study was to employ a portable 3D scanner (Artec 3D scanner EVA) to determine three commonly encountered regions of interest (ROI) around the knee.

Population
12 healthy volunteers (24 legs, 7 women, 5 men) were included in this study. The patient cohort showed a mean age of 27.1 years (SD±3), a mean weight of 70 kg (SD±13) and a mean height of 171 cm (SD±8.8). Participants who documented injuries or any other functional disorders regarding knee, or ankle were excluded from the study. Each subject completed a standardized questionnaire (age, height, weight, gender, supporting vs. free leg) and an informed consent was obtained prior to the procedure. The study was performed according to the guidelines provided by the Declaration of Helsinki and was approved by the university ethical committee (STUDY NUMBER 2019-475).

Study protocol and scanning procedure
All scans were recorded by the same examiner and took place in the same room and with the same scanner (Artec, Modell EVA). Before scanning, circumferences were indicated with a marker. Hereby, the tibial tuberosity was established as a beginning point and the circumferences were marked 20 cm proximal and 10 cm distal, subdivided in segments of 2.5 cm (12 Volumes-V, Fig 1).
Volunteers were seated and their full extended legs were placed on a rest table. Volunteers were asked to keep a natural foot position (90 degree angle). After that, the scanning procedure was started, and the examiner moved the scanner around the volunteer (Fig 2) until the knee as well as the surrounding ROI were completely recorded. All volunteers were instructed not to move during the scans. The ideal distance to perform the best scan was determined by the distance adjustment indicator within the Artec Studio 13 Software (Version 13, Artec Group, Luxembourg).
Each scanning took around 5.8 (SD ± 2) minutes. Three volume regions of interest (ROI, Fig 3) were defined. To evaluate the interobserver-reliability, all ROIs were determined by four different orthopaedic surgeons and compared to each other.
The distal femur was defined from the circumference of top of the patella to 10 cm proximal (Volume 1-Volume 4). The knee joint was defined from the top of the patella to tuberositas tibiae (Volume 5-Volume 8). The proximal tibia starts from tuberositas tibiae and ends 10 cm distal (Volume 9 -Volume 12). The mean distance from the tuberositas tibiae to the top of the patella in full extended legs in 12 cases was 11.4 cm (SD ± 1.8). The suprapatellar bursa is

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located proximal to the knee joint, between the prefemoral and suprapatellar fat pads. In most (~85%) people, the suprapatellar bursa communicates with the knee joint. The suprapatellar bursa does not communicate with the knee joint in~15% of people, remaining separated by an embryonic septum [5,6]. However, we included the suprapatellar bursa in the distal femur.

Statistical analysis
Statistical analysis was performed using GraphPad Prism (Version 8.1.2, San Diego, California, USA). Data was first tested for normality using D'Agostino-Pearson normality test. Data which showed no normal distribution were further tested using Wilcoxon-Mann-Whitney signed-rank test to compare the volume differences between the left and right leg. Pvalues � 0.05 were considered significant. An a priori power analysis was performed (G � Power Version 3.0.10, Franz Faul, University of Kiel, Germany). This resulted in a sample size of 10 for a power of 80% with a p value of 0.05 determining significance.

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Perioperative volumetric assessment of knee volumes with a portable 3D scanner

Results
The collective consisted of 7 women and 5 men (mean age 27.1 years, range 23 to 34 years). Leg volumes (volume 1-volume 12) ranged from 2.77 liter to 6.11 liter. Each 3D measurement required around 5.8 ± 2 minutes. The results of reproducibility of these three ROIs by four raters were highly reliable (intraclass correlation coefficient (ICC) = 0.90, 95% confidence interval: 0.87-0.93)

Comparison left vs right leg
Overall leg volume and volumes of the ROIs (I. distal femur, II. knee joint, III. proximal tibia) were compared between right and left leg (Fig 4).

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The Wilcoxon-Mann-Whitney test showed no significant differences of the volumes between the right and left leg (Overall leg volume: p = 0.79, I. distal femur: p = 0.34, II. knee joint: p = 0.62, III. proximal tibia: p = 0.38, Fig 3).
No significant volume differences between supporting and free leg were found (p>0.05).

Discussion
Objective assessment of soft tissue and swellings is a substantial challenge to orthopedic and trauma surgeons. Incorrect characterization may affect the timing of surgery and is higly correlated with soft tissue complications. No reliable objective measurement method exists for determining peri operative swelling of the limbs. Although certain methods have been described including bioelectrical impedance, tape measurement and water displacement methods [3,[7][8][9][10][11], they are all difficult to perform on a trauma patient. In particular, water displacement and tape measurement albeit reliable [4,12,13] cannot be used with open wounds and is both time-consuming and cumbersome. Moreover, the water displacement method provides zero information about the shape of the injured extremity [4,7,14]. For all of the reasons, perioperative assessment of soft tissue swelling is still performed in a subjective manner and varies among surgeons. An ideal method for volume assessment of the limbs of injured patients should be valid, objective, reliable, non-invasive, fast and preferably without the use of radiation. Especially in trauma surgery, preoperative volume comparison to the contralateral healthy limb is crucial. It is believed that a novel 3D scanner may offer substantial advantages and compete with existing methods [3,4]. Koban et al. tested the validity of the portable Artec Eva 3D scanner for medical purposes and showed a significant correlation to the water displacement method [3,7]. Seminati et al. analyzed the mean percentage error of the Artec Eva scanner in comparison to the water displacement method and identified only a mean error of 1.4% [15].
However, there is a lack of information concerning the volume variability between contralateral limbs in healthy subjects. Therefore, evidence -based assessment of the volume of an injured knee in comparison to the contralateral healthy side is not possible. In contrast to the water displacement method, portable 3D scanners are capable to analyze a specific region of interest (ROI) and detect differences to the unharmed contralateral limb in this ROI. This is particularly helpful for staged soft tissue management in severely injured patient (e.g.distal femur fractures, proximal tibia fractures or intraarticular knee injuries). To assess soft tissues for typical injuries in orthopedics and trauma surgery, it was mandatory to define three typical ROI around the knee (I. distal femur, II. knee joint, III. proximal tibia). In Accordance to previous studies, landmarks for the ROI with the highest inter-and intraobserver reliability were chosen [16]. Our results showed no significant differences of the overall volume and all three ROI between right and left leg. Accordingly, the selected ROIs seem to be valid for assessing and comparing injured limbs to the healthy contralateral side. In the present study, scanning time for both knees was approximately 5.8 (SD± 2) minutes and not significantly longer than for other medical purposes like assessment of lymphedema in previous studies [4]. Therefore, our data are in line with previous studies and suggest, that soft tissue assessment around the knee with the portable Artec Eva 3D scanner could be faster than conventional tape measurement and water displacement measurement [4] This study has several limitations. One limitation of this study is the small sample size of 12 healthy participants. The portable Arctec 3D scanner has a resolution of 0.1mm and previous studies with a similar sample size have shown a mean percentage error of 1.4% when compared to other methods [15]. Another limitation is that only one specific camera system was used for volumetric assessment. The Artec Eva 3D scanner has been previously used for medical purposes and showed a significant correlation to the goldstandard volumetric assessment [3,7,15]. A limitation of this technology is that scanning of a patient in a splint is not possible. Patients with an external fixation can be scanned as Artec software offers the ability to to substract external fixation after the scanning process has been completed. Finally, only leg volume of healthy volunteers was measured. Aim of this pilot study was to determine ROI of the knee that can be used for the most types of injuries. Therefore, our results are going to use for future studies to evaluate injured patients and therapeutic strategies over time.

Conclusion
This study demonstrates that portable 3D scanning may be an accurate and reliable tool for orthopedics trauma surgeons. Improved soft tissue management may improve the outcomes of severely injured knees. Subsequent studies that include injured patients are required to validate the ROIs.